1. Field of the Invention
The present invention relates to a multiple chamber wafer processing system and, more particularly, to a method and apparatus for scheduling processing for semiconductor wafers within a multiple chamber semiconductor wafer processing tool having a multiple blade wafer transfer mechanism.
2. Description of the Prior Art
Semiconductor wafers are processed to produce integrated circuits using a plurality of sequential process steps. These steps are performed using a plurality of process chambers. An assemblage of process chambers served by a wafer transport robot is known as a multiple chamber semiconductor wafer processing tool or cluster tool.
FIG. 1 depicts a schematic diagram of an illustrative multiple chamber semiconductor wafer processing tool known as the Endura System manufactured by Applied Materials, Inc. of Santa Clara, Calif. Endura is a trademark of Applied Materials, Inc. of Santa Clara, Calif. This tool can be adapted to utilize either single, dual, or multiple blade robots to transfer wafers from chamber to chamber.
The cluster tool 100 contains, for example, four process chambers 104, 106, 108, 110, a transfer chamber 112, a preclean chamber 114, a buffer chamber 116, a wafer-orienter/degas chamber 118, a cooldown chamber 102, and a pair of load lock chambers 120 and 122. Each process chamber represents a different stage or phase of semiconductor wafer processing. The buffer chamber 116 is centrally located with respect to the load lock chambers 120 and 122, the wafer orienter/degas chamber 118, the preclean chamber 114 and the cooldown chamber 102. To effectuate wafer transfer amongst these chambers, the buffer chamber 116 contains a first robotic transfer mechanism 124, e.g., a single blade robot (SBR). The wafers 128 are typically carried from storage to the system in a plastic transport cassette 126 that is placed within one of the load lock chambers 120 or 122. The robotic transport mechanism 124 transports the wafers 128, one at a time, from the cassette 126 to any of the three chambers 118, 102, or 114. Typically, a given wafer is first placed in the wafer orienter/degas chamber 118, then moved to the preclean chamber 114. The cooldown chamber 102 is generally not used until after the wafer is processed within the process chambers 104, 106, 108, 110. Individual wafers are carried upon a wafer transport blade 130 that is located at the distal end of the first robotic mechanism 124. The transport operation is controlled by the sequencer 136.
The transfer chamber 112 is surrounded by, has access to, the four process chambers 104, 106, 108 and 110, as well as the preclean chamber 114 and the cooldown chamber 102. To effectuate transport of a wafer amongst the chambers, the transfer chamber 112 contains a second robotic transport mechanism 132, e.g., a dual blade robot (DBR). The mechanism 132 has a pair of wafer transport blades 134A and 134B attached to the distal ends of a pair of extendable arms. The blades are used for carrying the individual wafers. In operation, one of the wafer transport blades (e.g., blade 134A) of the second transport mechanism 132 retrieves a wafer from the preclean chamber 114 and carries that wafer to the first stage of processing, for example, a physical vapor deposition (PVD) stage within chamber 104. If the chamber is busy, the robot waits until the processing is complete and then swaps wafers, i.e., removes the processed wafer from the chamber with one blade (e.g., blade 134B) and inserts a new wafer with a second blade (e.g., blade 134A). Once the wafer is processed and PVD stage deposits material upon the wafer, the wafer can then be moved to a second stage of processing, and so on. For each move, the transport mechanism 132 generally has one blade carrying a wafer and one blade empty to execute a wafer swap. The transport mechanism waits at each chamber until a swap can be accomplished.
Once processing is complete within the process chambers, the transport mechanism 132 moves the wafer from the process chamber and transports the wafer to the cooldown chamber 102. The wafer is then removed from the cooldown chamber using the first transport mechanism 124 within the buffer chamber 116. Lastly, the wafer is placed in transport cassette 126 within the load lock chamber 122.
More generally, a cluster tool contains n chambers, denoted by C.sub.1, C.sub.2, . . . , C.sub.n, one or more transfer chambers (robots), and one or more load locks. The exact arrangement of chambers is known as the configuration. A wafer W.sub.a to be processed is taken from a load lock, placed successively into various chambers C.sub.j, e.g., C.sub.1, C.sub.2, . . . , C.sub.k, respectively, and then returned to a load lock. As such, the wafers "trace" through the tool is represented by ##STR1## where W.sub.a visits chambers C.sub.1 and C.sub.2. Denotation C.sub.j and C.sub.j+1 means that a wafer has to visit chamber C.sub.j+1 after visiting chamber C.sub.j. Note that a wafer's trace does not have to involve all chambers in the configuration.
As defined in equation 1, a wafer's trace is the trajectory of a particular wafer through the cluster tool; that is, it traces the order in which the chambers are visited by a wafer. This should be distinguished from the term "processing sequence" which is the order of applying processes (recipes) to a wafer. If more than one chamber executes the same process (parallel chambers), a given processing sequence may be satisfied by several different traces. A processing sequence is known ahead of time and is a part of a computer program known as a "sequencer" which schedules the movement of wafers through the cluster tool; describes processes to be applied to the wafers while in the various chambers; describes processes a chamber is subjected to while in clean mode; describes conditions for the status change of a chamber (e.g., how many wafers or how much time before the clean process must be performed); and the like. An alternative term for a sequencer is a "router".
A wafer which completes its processing sequence and is returned to the load lock is said to be processed by the tool. Roughly speaking, a tool's throughput is the number of wafers processed by the tool per unit of time. That is, if the tool needs T seconds to process N.sub.t wafers, then EQU Throughput=N.sub.t /T (2)
The tool's throughput measured in the interval [0,T]. There are many ways to improve the tool's throughput for a given processing sequence. However, one important improvement is the use of a dual blade robot in one or more of the transport mechanisms. Generally, the second blade is fixed at a position 180.degree. opposite the first blade within a transport mechanism, e.g., as shown in FIG. 1. Alternatively, one blade may be positioned above the other blade and the two blades are either fixed with respect to one another (i.e., an over-under fixed (OUF) robot) or move independently with respect to one another (i.e., an over-under independent (OUI) robot). In any of those configurations, the scheduling processes used to fulfill a given process sequence differ from those scheduling processes used for a single blade transport mechanism.
Heretofore, a cluster tool processed wafers in the sequence they were received. This can lead to a less than optimal throughput.
Therefore, a need exists in the art for a scheduling routines that efficiently control a multiple blade robot within a cluster tool.